1. Technical Field of the Invention
This disclosure generally relates to an optical device such as for example, but not limited to an image capturing device or a beam projector, and more particularly, to a focusing apparatus of a subminiature optical device which can be mounted on a small device such as a portable terminal.
2. Description of the Related Art
Optical devices may be classified into image capturing devices and image projecting devices such as beam projectors. The image capturing device includes an image sensor, while the image projecting device includes a light source and a liquid crystal panel, but they include in common a lens group including at least one lens in an image input or output path. Optical devices are typically utilized within an independent device such as a camera or a beam projector. Recently, with the wide spread use of portable terminals such as a mobile communication terminal, the optical device has also been mounted on the portable terminal.
A camera mounted on a portable terminal has been used to capture a still image or a moving image, and recently, the camera has been used to realize virtual reality with local information, neighboring shopping center information, etc., through the portable terminal. There has been an increased desire and need to attempt to mount an image projecting device, such as a beam projector, which has been realized and portable terminals equipped with a beam projector function have now released in the market. The beam projector can implement a large screen relative to its size, thus being variously used in conferences, performances, etc. In large beam projectors which are generally provided as independent units and installed in public spaces such as halls, conference rooms, and so forth, a position sensor for sensing a position of a focus is installed, however in a miniaturized device such as a portable terminal, it is difficult to install the position sensor due to special restrictions. Therefore, for installation in a limited space, a focusing apparatus capable of performing stable focusing without the need to install the position sensor is required.
FIG. 1 is a plane view of a focusing apparatus 100 for an optical device according to an embodiment of conventional prior art, and FIG. 2 is a view for describing a focusing operation of the focusing apparatus 100 shown and described with respect to FIG. 1.
As shown in FIGS. 1 and 2, a focusing apparatus 100 for an optical device, specifically, a beam projector according to an embodiment of conventional prior art performs focusing as a lens barrel 101, receiving at least one lens, linearly moves in a direction along the optical axis. To guide linear movement of the lens barrel 101, the focusing apparatus 100 includes guide rods 119. The guide rods 119 are positioned at both sides of the lens barrel 101 and are coupled with guide members 111a and 111b formed at both sides of the lens barrel 101 in such a way to go through the guide members 111a and 111b. The guide members 111a and 111b slide together with the lens barrel 101 in a direction in which the guide rods 119 extend.
As a driving means for linearly moving the lens barrel 101, a dial structure or a lever structure for directly providing a driving force by a user may be used, or a structure using a driving motor such as a step motor or a servo motor may be used. In the embodiment shown in FIG. 1, to provide the driving force for linearly moving the lens barrel 101, the focusing apparatus 100 includes a driving motor 121, a lead screw 102 rotated by the driving motor 121, and a bracket 123 for mounting the driving motor 121 and at the same time, for rotatably installing the lead screw 102. The focusing apparatus 100 also includes a carriage 103 screw-coupled with the lead screw 102.
The carriage 103 linearly moves in a longitudinal direction of the lead screw 102 as the lead screw 102 rotates, and as a link arm 131 formed on the outer circumferential surface of the carriage 103 is interfered with by at least one of the guide members 111a and 111b, the guide members 111a and 111b also linearly move together with the carriage 103. Therefore, as the driving motor 121 operates, the lead screw 102 rotates, linearly moving the carriage 103, such that the carriage 103 linearly moves the guide members 111a and 111b and the lens barrel 111 through the link arm 131.
However, in the focusing apparatus structured as described above, due to manufacturing and assembly tolerances between parts, interworking between the link arm and the guide members is not precise. That is, due to manufacturing and assembly tolerances between parts, there is a difference generated between a movement distance of the carriage based on the operation of the driving motor and an actual movement distance of the lens barrel. Moreover, due to the hysteresis caused by accumulated manufacturing and assembly tolerances between parts, during focusing, in spite of switch of the moving direction of the carriage, the lens barrel maintains a stationary state, such that even if the carriage moves in a direction and then returns to its initial position, the lens barrel fails to return to its initial position. Furthermore, due to many restrictions in installation of the position sensor of the lens barrel in a miniaturized device such as a portable terminal, a difference between a movement distance of the carriage and a movement distance of the lens barrel makes it difficult to control the driving motor. In addition, since the link arm and the guide member does not contact in a sufficient area to provide accurate rigid support, tilt of the lens barrel occurs during movement of the lens barrel as shown in FIG. 2, such that the lens barrel may become misaligned with respect to the optical axis.